a. Field of the Invention
The present application claims the benefit of priority to United Kingdom Application No. 0417765.5 filed Aug. 10, 2004, hereby incorporated by reference in its entirety.
This invention relates to a well casing straddle assembly, that is to say an assembly comprising a liner and at least one packer/seal at the top and at the bottom of the liner so that the assembly can be set in a well casing to isolate a length of the casing from the well bore. A straddle assembly is typically used to repair defective casing or to isolate openings in the casing (for example perforations or window openings) from the well bore.
b. Description of Related Art
The use of straddle assemblies is well known in the oil industry. A typical straddle assembly is described in U.S. Pat. No. 3,948,321 and comprises a packer/seal assembly located at both the top and the bottom end of a length of liner. In use, the assembly is run into a well and the packer/seal assemblies are set to hold the straddle assembly in place and provide a fluid tight seal between the assembly and casing at the top and bottom of the assembly.
In general, it is desirable for the inside diameter of the assembly to be as large as possible relative to the inside diameter of the casing in which it is set and to this end the packer/seal assemblies are designed to provide the maximum available internal diameter.
In many applications it would be desirable to have more than one packer/seal assembly at the top and/or at the bottom of the straddle assembly. Typically, in many applications it would be desirable to have two packer/seal assemblies at the top of the straddle assembly and two packer/seal assemblies at the bottom of the straddle assembly. However, attempts to implement designs using a total of four packer/seal assemblies have not been entirely satisfactory. The problem arises from the method which is used to set the packer/seal assemblies. In the prior art, a setting tool is deployed within the straddle assembly and latched onto a suitable collar provided at the lower end of the assembly. Means are provided on the setting tool adjacent the upper end of the assembly to provide a reaction surface against which the straddle assembly could be pulled by the setting tool. Once movement of the upper end of the straddle assembly had been arrested, continued action of the setting tool applies an axially compressive force to the straddle assembly to set the packers. Although all four packers would, theoretically, have equal setting forces, in practice manufacturing tolerances, wear, well conditions, etc. mean that the packers set in an order which could not be predicted. If, in fact, the packers nearest the top and bottom of the assembly set before the packers nearer the centre of the assembly, further movement of the setting tool was arrested and the packers nearer the centre of the tool were not, in fact, set at all. This was obviously highly undesirable. Although it would be possible, in theory, to devise a setting tool which could selectively set each of the packers in turn, such a design would be complex and difficult to operate under field conditions.
Accordingly, it is an object of the present invention to provide a well casing straddle assembly having a plurality of packer/seal assemblies in which the sequence of setting of the packer/seal assemblies can be selected, thereby allowing optimum setting.
The ability to select the setting order is particularly critical in a straddle assembly of the type which includes two or more packer/seal assemblies at the top of the straddle assembly and a further two or more packer/seal assemblies at the bottom of the straddle assembly. However, it should be understood that the ability to select the setting order may be of significant utility even in designs in which there is only one packer/seal assembly at each end of the straddle assembly. In particular, the ability to set the packer at the end of the straddle assembly which is fixed relative to the well casing during the setting procedure, before setting the packer/seal assembly at the other end of the straddle assembly, is highly desirable since it avoids the possibility of the packer/seal assembly at the moveable end of the straddle assembly being set prematurely and thereby resisting further movement of the setting tool.